Images of the interiors of bodies may be acquired using various types of tomographic techniques, which involve recording and measuring radiation from tissues and processing acquired data into images.
One of these tomographic techniques is positron emission tomography (PET), which involves determining spatial distribution of a selected substance throughout the body and facilitates detection of changes in the concentration of that substance over time, thus allowing to determine the metabolic rates in tissue cells.
The selected substance is a radiopharmaceutical administered to the examined object (e.g. a patient) before the PET scan. The radiopharmaceutical, also referred to as an isotopic tracer, is a chemical substance having at least one atom replaced by a radioactive isotope, e.g. 11C, 15O, 13N, 18F, selected so that it undergoes radioactive decay including the emission of a positron (antielectron). The positron is emitted from the atom nucleus and penetrates into the object's tissue, where it is annihilated in reaction with an electron present within the object's body.
The phenomenon of positron and electron annihilation, constituting the principle of PET imaging, consists in converting the masses of both particles into energy emitted as annihilation photons, each having the energy of 511 keV. A single annihilation event usually leads to formation of two photons that diverge in opposite directions at the angle of 180° in accordance with the law of conservation of the momentum within the electron-positron pair's rest frame, with the straight line of photon emission being referred to as the line of response (LOR). The stream of photons generated in the above process is referred to as gamma radiation and each photon is referred to as gamma quantum to highlight the nuclear origin of this radiation. The gamma quanta are capable of penetrating matter, including tissues of living organisms, facilitating their detection at certain distance from object's body. The process of annihilation of the positron-electron pair usually occurs at a distance of several millimeters from the position of the radioactive decay of the isotopic tracer. This distance constitutes a natural limitation of the spatial resolution of PET images to a few millimeters.
A PET scanner comprises detection devices used to detect gamma radiation as well as electronic hardware and software allowing to determine the position of the positron-electron pair annihilation on the basis of the position and time of detection of a particular pair of the gamma quanta. The radiation detectors are usually arranged in layers forming a ring around object's body and are mainly made of an inorganic scintillation material. A gamma quantum enters the scintillator, which absorbs its energy to re-emit it in the form of light (a stream of photons). The mechanism of gamma quantum energy absorption within the scintillator may be of dual nature, occurring either by means of the Compton's effect or by means of the photoelectric phenomenon, with only the photoelectric phenomenon being taken into account in calculations carried out by current PET scanners. Thus, it is assumed that the number of photons generated in the scintillator material is proportional to the energy of gamma quanta deposited within the scintillator.
Inorganic scintillators used in the PET technique, usually 0.5 cm×0.5 cm in cross-section and in the order of 1 cm in length are usually arranged in blocks of the size of several centimeters. Photomultipliers are attached to the back side of each block so as to convert the received light pulses into electrical pulses. Such an assembly allows to determine the position of reaction of the gamma quantum with the accuracy equal to the size of the smaller element. In further analysis, when reconstructing the image, an assumption is made that the gamma quantum was absorbed in the centre of the element. Lack of information on the depth of interaction (DOI) is one of the reasons that limit the resolution of acquired images. The further the distance of the annihilation position from the scanner's axis and the thicker the scintillator blocks, the higher the image deformations; therefore, determination of DOI is particularly important for the improvement of whole body imaging by improving the resolution far from the instrument axis and increasing the gamma quantum capture capabilities by allowing application of thicker scintillators.
Polymeric scintillators with relatively short decay times of ca. 2 ns are also known, being well suitable for measuring the particle detection times. They are commonly used in nuclear physics and elementary particles physics to record energy losses and transition moments of charged particles. Most commonly, scintillators are shaped as strips of rectangular cross-section. Light pulses (scintillations) that accompany interactions of charged particles, such as electrons, or neutral particles, such as gamma quanta, with scintillator material are recorded by a pair of photomultipliers optically connected to the end of each strip.
PET scanners making use of polymeric scintillators for detection of gamma radiation and methods for determination of the positions of reaction of gamma quanta in the applied detection systems are known in literature.
The PCT application WO2011/008119 disclosed a strip scanner based on polymeric strip scintillators read-out at both ends by a pair of photomultipliers. The position of interaction of the gamma quantum in the scintillator material is measured along the length of the strip and calculated on the basis of the differences in times of propagation of light pulses recorded by the pair of photomultipliers. Assuming a very high resolution of 100 ps (value expressed as full width at half maximum [FWHM]), for the light pulse propagation time difference and considering the fact that the light pulse propagation speed is lower than the speed of light in vacuum by about a factor of two, the position resolution of the scintillator used in the solution is at the level of 0.75 cm (FWHM).
In the PCT application WO2011008118, disclosing a matrix tomography scanner, a polymeric scintillator plate was used in the detector system, with light signals being detected along the periphery as well as on one side of scintillator plate by a matrix of photomultipliers. The recorded times of pulse propagations and the amplitudes of the pulses facilitate determination of the position and times of gamma quanta interacting with scintillators.
The aforementioned methods to determine the positions of the reactions of the gamma quanta used in acquisition of positron emission tomography images allow a conclusion that there is a continuous need for the development of new methods allowing for more precise determination of the positions of the gamma quanta reactions as well as of devices allowing for better positional resolution capabilities of polymeric scintillators.
It would be expedient to develop a device allowing for precise determination of the position of the reaction of the gamma quanta in positron emission tomography with simultaneous improvement of imaging via ameliorating positional resolution capabilities of the detector system.